Electrochemical machining using a film forming electrolyte including amine alcohols



United States Patent 3,421,987 ELECTROCHEMICAL MACHINING USING A FILM FORMING ELECTROLYTE INCLUDING AMINE ALCOHOLS Mitchell A. La Boda, East Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Filed Oct. 23, 1965, Ser. No. 504,108 US. Cl. 204143 3 Claims Int. Cl. B23 1/16 ABSTRACT OF THE DISCLOSURE Electrochemical erosion-inhibiting film-forming electrolytes for electrochemically machining ferrous metals, consisting essentially of aqueous solutions of alkali metal chlorides and amines of low molecular weight alcohols.

This invention relates to electrochemical machining processes and, more particularly, to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes, cavities and contoured surfaces have been developed and are more generally classified into one of two basic categories, the first being electrochemical machining and the second electrolytic grinding, a specialized application of the first. Electrolytic grinding is essentially an electrochemical deplating process which can be used on virtually any electrically conductive material. It is generally adapted to metal removal operations comparable to those performed by cutoff wheels, saws, and grinding or milling machines and the like, and uses equipment similar to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action.

A particular version of an electrolytic grinding process is characterized by a flow of electrolyte between the workpiece and a rotating grinding cathode wheel. The rotating cathode wheel comprises a conductive metal matrix having a plurality of nonconducting abrasive particles imbedded therein to provide nonconductive spacing between the workpiece and the cathodic matrix. Electric current is passed through the electrolyte to dissolve the anodic surfaces of the workpiece and the imbedded particles of the wheel abrade the surface to remove any irregularities resulting from nonuniform erosion or reaction product build-up.

While aqueous solutions of individual inorganic salts, such as nitrates, cyanides, carbonates, hydroxides and nitrites have been used as electrolytes in electrochemical machining and grinding processes, none has offered any significant advantage over the now well accepted aqueous sodium chloride solution most commonly used today. However, regardless of what salt is chosen, an apparent problem with electrochemical machining and grinding processes using that salt singularly or in combination is overcut, which is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. Such anodic dissolution can occur in areas which are fairly well removed from the cathode. This overcut, or cutting in low current density areas which are bathed in the electrolyte but substantially removed from the cathode, has been substantially reduced in the prior art by the use of costly and time-consuming masking operations which isolate the areas to be machined by protecting the surrounding areas from the erosive effect of the electrolyte. These masking operations are frequently quite involved and require a high degree of skill to insure a satisfactory product. Likewise, additional steps subsequent to the machining stepsare required to strip the workpiece of the mask. Additionally,

3,421,987 Patented Jan. 14, 1969 the prior art has attempted to reduce overcut by designing special purpose electrodes and machines to meet individual and specialized machining requirements.

By my invention I have at least reduced, and in most cases actually eliminated, the need for recourse to the prior arts attempted resolutions.

It is, therefore, an object of my invention to provide a self-masking electrolyte for ECM.

It is a further object of my invention to provide an additive for existing ECM electrolytes which inhibits anodic dissolution.

It is a further object of my invention to effect a sharply contoured machining by a process utilizing a basic aqueous electrolyte containing known salts and improving same by adding thereto a compound which upon reaction with the workpiece forms a film thereover, which film inhibits or stops off electrolytic action or anodic dissolution, and subsequently removing said film in selected areas to permit continued machining exclusively in those areas.

It is a further object of my invention to effect a sharply contoured electrochemical machining by a process utilizing aqueous electrolytes containing known salts and an additive consisting of an amine of a low molecular weight alcohol, particularly primary, secondary and tertiary ethanolamine.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

My invention briefly stated involves adding an amine of a low molecular weight alcohol to substantially neutral ECM electrolytes. By an amine of a low molecular weight alcohol, I mean up to and including the butanolamines. When added to ECM electrolytes the additives of my invention, and especially the ethanolamines, create a solution which effectively forms a heavy adherent film over the surface of the workpiece. The film retards or substantially eliminates electrochemical erosion in those areas protected by the film. In a particular application a chloride electrolytic grinding electrolyte is modified in accordance with my invention. The film formed is subsequently abraded away in those areas where electrochemical machining is to continue, hence presenting a limited uninhibited surface to unrestricted electrochemical action.

My experience has been that using the additives of my invention, I can successfully machine metal samples ranging from the softer low carbon steels (SAE 1008) to the harder low alloy steels (SAE 5l6 0H). However, the additive of my invention will be likewise applicable and effective for ferrous alloys :wherein the iron content is 50% or more.

While the preferred electrolytes comprise either about 4.3 to 18.43% by weight of monoethanolamine or about 3.4 to 9.1% by weight of triethanolamine or combinations thereof, I have found that effective electrolytes can be compounded using from 2.2 to 42.0% by weight of ethanolamine, the balance being a selected concentration of an aqueous sodium chloride solution wherein the concentration of the sodium chloride may vary from dilute to saturated. In this connection the lighter alkali metal (lithium, sodium and potassium) chlorides are preferred because they produce relatively neutral pHs, do not plate out or have a deleterious effect upon the cathode, and represent a source of inexpensive material. Likewise, while electrolytes dilute as to chloride ion are operative, as a practical matter there are no significant advantages to operating at the lower concentrations and, in fact, it is less desirable to do so when considering such factors as solution conductivity and the like. Hence solutions in the order of 15-20% NaCl are preferred.

While there is every indication that the soluble higher aliphatic alcohol amines, i.e. pentanolamine and above,

will be equally operative, their availability and cost have rendered their use impractical.

I have been successful in operating these electrolytes at voltages up to 40 volts and anode current densities from 5 to 500 amperes per square inch. However, for each particular additive concentration there appeared to be a maximum current density above which practical control over the machining was lost.

The exact explanation for this was not ascertained, but

Tube length decrease per unit time was used to determine metal removal rates. The tube ends were compared with those produced by electrochemically grinding similar samples under the same conditions with additive-free electrolytes. Similar tests were conducted with non-tubular stock material.

The following table represents the results of experiments conducted in accordance with the procedure outlined above.

TABLE Additive Cone. (mls.) Volts (v.) Current density Metal removal Sample SAE Results (a.s.i rate, in./min.

25 3. 8 200 0. 029 1018 -Very square cut. Little burr left. 25 8.4 540 0.063 1018 Sornje loss of corner; little burr on trailing si e.

50 4. 250-290 0. 026 1018 Slight evidence of inhib.

50 8. 540 0.064 1018 Slight evidence oi inhib.

200 4.0 290 0. 008 1018 8stopped. Mechanical component 200 4. 0 260 0. 001 1018 ECM stopped.

200 4. 5 115-155 0. 011 1018 ECM stopped.

200 9.5 230 0.027 1018 ECM slowed to fraction of N aClsolution 4.1 230 0.034 1018 Inhib. indei.

25 0. 011 1018 Mechanical cutting component.

25 8. 7 460 0. 066 1018 No ll'lllll).

4. 2 250 0.032 1018 Strong inhib. Little burr lelt.

50 8.7 400 0.066 1018 Too little inhib. at this current density. 100 4. 2 250 0. 020 1018 Front inhib. increased; side inhib. good. 100 8.8 425 0.058 1018 Front inhib. increased; side inhib. good. 100 4. 5 100 0.012 510011 Strong inhib. 100 9. 2 260 0.028 5160H Insuflicient inhib. at this current density. 100 4. 3 -100 0. 010 4620 Inhib., but not strong. 9. 3 260-285 0. 025 4620 Inhib., but not strong. 200 4. 5 75-100 0. 008 5160H Strong inhib.; poor surface finish. 200 4. 5 75 0. 005 4620 Strong inhib.; poor surface finish. 250 4.8 75 0.007 5160B. Strong inhib.; finish improved. 250 4. 8 75 0. 005 4620 Strong inhib.; finish improved. 300 4. 7 75 0. 006 5160B Excellent inhib.; surface finish improved. 300 4. 7 50-75 0. 004 4620 Very good inhib.; finish improving. 400 4. 7 40 0. 0035 5160]?! Excellent inhib.; excellent finish. 400 9. 75 130 0.013 510011 Excellent inhib.; excellent finish. 400 4. 5 25 0.003 4620 Very good inhib.; good finish. 400 9. 5 0.011 4020 Very good inhib. and finish. 400 15. 2 230 0.019 4620 Very strong inhib.; good surface finish. 500 4. 6 17 0.0025 516011 Perfect inhih.; ECM stopped. 500 9. 8 0.011 51001-1 Eiirlcclllcnt tip. Excellent inhib. and

ms 1. 500 15. 2 0. 017 5160H Very good tip; good inhib. and finish. 500 4. 7 12 0.004 4620 Perfect inhib.; acceptable finish. 500 9. 8 100-130 0.010 4620 Exxcellent tip; unset. finish. 500 15.2 230 0. 016 4620 Excellent inhib.; good finish. 000 4. 7 17-25 0.005 5100H Very good tip; strong inhib.; good finish. 000 9. 8 90-100 0. 009 516011 Very good inhih. and finish. 600 15.2 190 0. 018 510011 Excellent tip, inhib. and finish. 600 20.1 200 0.020 5160H Unstlit. inhib. and finish-best at this 4 v0 tage. 600 4.7 17 0.002 4620 Good inhib. and finish. 600 9. 8 90-100 0.0075 4620 Good tip, inhib. and finish. S00 4. 7 4-8 0.003 5160H Very strong inhib., ECM about stopped. 800 9. 8 63 0. 0007 51601-1 Very good inhib. and finish. 800 9. 8 63-100 0.009 516011 Very good inhib. 800 4.7 10 0.003 4620 Very good tip-not improved over lower concentrations.

800 9. 8 100-125 0.0085 4620 Very good inhib.

50 4. 0 50 0.010 5100H Fair inhib. and finish.

50 9. 0 300 0. 030 516011 Inhib. improved over lower current density samples.

I Mls. of amino alcohol added to 1 liter 18% NaCl solution.

MEA =l\1onoethanolamine, TEA =Triethanolamine, 'IPA =Triisopropanolamine (mix).

it is felt that the film formed may be breaking down at the higher current densities, much like has been disclosed in my copending patent application Ser. No. 504,133, filed Oct. 23, 1965.

Generally speaking, tests were conducted utilizing a system wherein steel tube samples were brought up to a rotating sintered bronze diamond impregnated wheel. A gravity feed system kept the samples at the face of the wheel at all times. The feed system was such that an adjustable weight provided the capability of varying the pressures at which the samples would engage the wheel. It was found that to properly evaluate the inhibitive effects of my additives a minimum workpiece-to-wheel pressure should be employed in order to reduce the mechanical cutting component of the abrasive wheel. A room temperature electrolyte was pumped at a pressure of 9 psi. through a bore in the workpiece and into the gap between the cathode and the workpiece at a rate of 0.25 gallon per minute. This gap was held constant by the spacer effect of the nonc nd ctive diamond chip Other than abrasive means for the local removal of the inhibiting film of my invention may be employed. Among the possibilities might be localized breakdown under high current densities, localized flushing, and/ or a variety of sophisticated variations of these and others. Therefore, though my invention has been described in terms of certain preferred embodiments, it is to be undertsood that others may be adapted and that the scope of my invention is not limited except by the appended claims.

I claim:

1. A process for electrochemically machining a ferrous metal comprising the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathode electrode adjacent to but closely spaced from said metal so as to form a gap therebetween, flowing through said gap a substantially neutral electrolyte consisting essentially of an aqueous solution of at least one alkali metal chloride and at least one amine of a low molecular weight alcohol in concentrations of from about 2.2 percent by weight to about 42.0 percent by weight to IOrm, an electrochemical erosion inhibiting film on said metal, passing References Cited current through said cell and removing from selected areas the electrochemical erosion inhibiting film formed UNITED STATES PATENTS whereby electrochemical machining can continue in said 2 939 325 1960 Faust, 1 204 142 areas. 5

2. The process as claimed in claim 1 wherein said ROBERT K. MICHALEK, Primary Examiner. alcohol is ethanolamine having a concentration of from about 2.2 to about 42.0% by weight. U.S. Cl. XJR.

3. The process as claimed in claim 2 wherein the con- 25 79.4 centration of said ethanolamine is 3.4 to 18.4% by weight. 10 

